When an x-ray image is obtained, there is generally an optimal angle between the x-ray source and the usual two-dimensional receiver or image detection device that records the image data. In most cases, the x-ray source preferably provides radiation in a direction that is perpendicular to the surface of the recording medium of the receiver. For this reason, large-scale radiography systems typically mount the radiation head containing the x-ray source at a specific angle relative to the recording medium. Orienting the head and the receiver typically requires a mounting arm of substantial size, extending beyond the full distance between these two components. Unwanted tilt or skew of the receiver is thus prevented by the mounting hardware of the imaging system itself.
With the advent of portable radiation imaging apparatus, such as those used in Intensive Care Unit (ICU) environments, a fixed angular relationship between the radiation source and two-dimensional radiation receiver usually is no longer imposed by the mounting hardware. Instead, a technician or operator is required to aim the radiation source toward the surface of the recording medium, providing as perpendicular an orientation as possible using a visual assessment. In computed radiography (CR) systems, the two-dimensional radiation receiver or image detection device itself is a portable cassette that stores the readable recording medium.
There have been a number of approaches to the problem of providing methods and tools to assist the technician with aiming the radiation source. One approach has been to provide mechanical alignment in a more compact fashion, such as that described in U.S. Pat. No. 4,752,948 entitled “Mobile Radiography Alignment Device” to MacMahon. A platform is provided with a pivotable standard for maintaining alignment between an imaging cassette and radiation source. However, complex mechanical solutions of this type tend to reduce the overall flexibility and portability of these x-ray systems.
Other approaches project a light beam in order to achieve alignment between source and receiver. Examples of this approach include U.S. Pat. No. 5,388,143 entitled “Alignment Method for Radiography and Radiography Apparatus Incorporating Same” and No. 5,241,578 entitled “Optical Grid Alignment System for Portable Radiography and Portable Radiography Apparatus Incorporating Same”, both to MacMahon. Similarly, U.S. Pat. No. 6,154,522 entitled “Method, System and Apparatus for Aiming a Device Emitting Radiant Beam” to Cumings describes the use of a reflected laser beam for alignment of the radiation target.
One solution for maintaining a substantially perpendicular relationship between the radiation source and the two-dimensional radiation receiver or image detection device is described in U.S. Patent Application Publication No. 2005/0058244 entitled “Portable Radiation Imaging System and a Radiation Image Detection Device Equipped with an Angular Signal Output Means” by Tanaka et al. This published application discloses an angular sensing device atop or along an edge of the image detection device. The angular sensing device sends a signal to adjust either the tilt angle of the image detection device or the orientation angle of the radiation source in order to maintain a perpendicular relationship of the image detection device to the radiation source. This same approach had previously been used in a number of X-ray products, such as the Siemens Mobilett XP hybrid portable X-ray source, for example, that used built-in tilt sensors.
Similar to the earlier approaches that also used tilt relative to gravity, the solution proposed by Tanaka et al. has limited value for achieving alignment between the image detection device and the radiation source. Measuring tilt with respect to gravity is suitable for one particular case: that is, where the image detection device is intended to be level and where the radiation source is to be perpendicular to the surface of the image detection device. In any other orientation, however, solutions of this type become increasingly less effective as the surface of the image detection device moves away from a perfectly level orientation. There is not enough positioning information with this type of solution for aligning the central ray of the radiation source with the normal to the surface of the image detection device. In the worst-case position, with the image-detection device in a near-vertical or vertical orientation, there is little or no information that can be obtained from tilt sensors as to whether or not the surface of the image detection device is perpendicular to the path of x-rays from the radiation source. Moreover, the particular solution proposed by Tanaka et al. does not assist the technician in making manual adjustments for tilt, but requires a more costly and trouble-prone system having motion control components.
Portable radiation imaging apparatuses allow considerable flexibility for placement of the CR cassette by the technician. The patient need not be in a horizontal position for imaging, but may be at any angle, depending on the type of image that is needed and the ability to move the patient for the x-ray examination. The technician can manually adjust the position of both the cassette and the radiation source independently for each imaging session. Thus, it can be appreciated that an alignment apparatus for obtaining the desired angle between the radiation source and the surface of the image detection device must be able to adapt to whatever orientation is best suited for obtaining the image. Tilt sensing, as has been conventionally applied and as is used in the device of Tanaka et al. and elsewhere, does not provide sufficient information on cassette-to-radiation source orientation, except in the single case where the cassette is level.
Thus, it is apparent that conventional alignment solutions may be workable for specific types of systems and environments; however, considerable room for improvement remains. Portable radiation imaging apparatus must be compact and lightweight, which makes the mechanical alignment approach such as that given in the '948 MacMahon disclosure less than desirable. The constraint to direct line of sight alignment reduces the applicability of reflected light based methods to a limited range of imaging situations. The complex sensor and motion control interaction required by Tanaka et al. would add considerable expense, complexity, weight, and size to existing designs, with limited benefits. Many less expensive portable radiation imaging apparatuses do not have the control logic and motion coordination components that are needed in order to achieve the necessary adjustment. None of these approaches gives the technician the needed information for making a manual adjustment that is in the right direction for correcting misalignment.
Significantly, none of the conventional solutions just described is particularly suitable for retrofit to existing portable radiation imaging apparatus. That is, implementing any of these earlier solutions would be, in practice, prohibitive for all but newly manufactured equipment and would have significant cost impact.
Yet another problem not addressed by many of the conventional solutions relates to the actual working practices of radiologists and technicians. A requirement for perpendicular delivery of radiation, given particular emphasis in Tanaka et al., is not optimal for all types of imaging. Rather, there are some types of diagnostic images for which an oblique (non-perpendicular) incident radiation angle is most desirable. For example, for the standard chest anterior-posterior (AP) view, the recommended central ray angle is oblique from the perpendicular (normal) by approximately 3-5 degrees. Conventional alignment systems, while they provide for normal incidence of the central ray, do not adapt to assist the technician for adjusting to an oblique angle.
Thus, it can be seen that there is a need for an apparatus that enables proper angular alignment of a radiation source relative to an image detection device for recording a radiation image.